They get the same number of players on the roster but by restricting the number of pitchers allowed on a roster you are now by default putting more stress on every pitching staff and thus increasing the likelihood of injury. This is especially relevant now given the way pitchers at all levels are conditioned (not the innings eaters of old). I may be doing this math wrong but assuming the average team carries 12.5 pitchers at any given time (some carry 12 others 13) and the average season is 1450 innings, you basically have to find 175 innings that were previously accounted for by that final roster spot or two (more than one inning per game). That is not insignificant and I dont think MLBPA would ever go for it.
This is an issue that will of necessity resolve itself through some kind of rule changes. It certainly would be ahistorical for pitchers to reverse the long-term trend by having them pitch deeper into games. The longer the training says five man rotation, 100 pitch (or less, for many starters) limit, it is increasingly unlikely -- I would say impossible -- that we will return to the era of starters going 250-300 innings per year or relievers working multiple innings per game. It is far more likely that limits on pitch counts will continue downward in a futile effort to cut down on arm injuries. I would not at all be surprised if 20 years from now baseball fans will wonder how a starter could ever be expected to throw as many as 90-100 pitches in a game.
There are several ways to deal with this. The easiest would be to expand rosters to 30 and have 15-17 man pitching staffs, with nobody throwing more than 50 or so pitches per outing. This would obviously greatly dilute pitching talent -- a 17 man staff would mean that over 100 pitchers who are not currently considered major league caliber would be on the rosters, greatly increasing offense, which MLB always seems to want.
A more radical change, but one that may have to be considered as pitchers are increasingly unable to handle big workloads, would be along the lines of reducing the game to seven or even six innings. High school teams play seven inning games, in part to reduce stress on young arms. I would not be shocked if a generation from now this concept is seriously contemplated. Also reducing the number of games played would help out. I would hate to see it come to that -- it would play havoc with the record books and history that most hardcore baseball fans love -- but again it would not be in the least surprising if this happened.
Another possible rule change, to blunt the practice of hitters driving up pitch counts, would be to say that after a hitter has two strikes, if he fouls off a second or third pitch, it counts as a strikeout.
Just a few ideas, none of which I would like to see implemented, but which will inevitably become part of the conversation as the drive to limit pitcher workloads moves ahead.
The days of a Juan Marichal-Warren Spahn pitching duel where each guy throws 16 innings and somewhere around 250 pitches (with no real after-effects) are not coming back. I would say unfortunately, but few would agree, which is fine by me.
And if you think those two were just lobbing it up there batting practice style and that's why they could throw so many pitches, you never saw them pitch. Those guys each threw from about eight different arm slots out of a high kick and Marichal alone must have had six different breaking pitches. There is no new pitch under the sun, only tweaks and refinements of what has been going on for at least a hundred years. Yes, there has been a progression, but it is only a matter of degree. And yes, there are more pitchers who throw 95-100 MPH than there were 50-60 years ago, but that is because they know they can let it rip because their time on the mound is limited. I have actually bothered to read Christy Mathewson's "Pitching In A Pinch" written over a century ago where he very frankly discusses saving his hardest stuff for key situations because he knew he was expected to pitch a complete game. Had he been expected to give five of his best innings, he would have approached the game differently.
Just an old guy's view after a couple of martinis.